Well, this is what I would do:Since people are actually posting on this... (which has never happened before)
How do you know which years to put when the question is like "x is in purple because it was built in between _____ and ______?") I never seem to get them right, and I always feel like the answer changes. Anyone know how to do 'em?
1) Usually, each map has a date where the data was field checked (or topo. was gathered)
2) Then, there is a date where the photorevision took place.
3) my answer would be the difference between these two dates.